6. DATA SOURCES › 6.8 PR/SM LPAR Concepts › 6.8.4 Interpreting PR/SM Data › 6.8.4.3 Logical Partition Interaction › 6.8.4.3.3 Logical Processor Dispatching and I/O Operations

6.8.4.3.3 Logical Processor Dispatching and I/O Operations

I/O operations are prioritized in a manner similar to
processor weighting.  Although the exact details of this
process are not documented, this prioritization is at least
partially based on the processor shares and on the relative
success of the LPAR dispatcher in accurately allocating the
central processor resources among competing partitions.  If
two partitions are given equal weight, the one that is a
little "behind" will receive higher priority than the one
that is a little "ahead."  This means that an I/O interrupt
for a partition with a higher priority may cause the central
processor to switch away from a lower priority partition in
order to process the interrupt.  This will obviously have a
detrimental effect on the lower priority partition.  In
addition, this switching effectively increases the rate of LP
dispatching, causing processor cache to be less effectively
managed and slowing the effective instruction execution rate.

Unfortunately, no mechanism exists for measuring this
effective lengthening of the I/O service time.  Although the
queue, pending, connect, and disconnect times are accumulated
for each device measured by RMF, the actual time it takes an
I/O operation to complete may be in excess of the sum of
these values if LPAR dispatching causes a delay in processing
an interrupt.

If the average time-slice value were available, it would be
possible to estimate the effects.  By using the duration of
the measurement interval, the sum of the dispatch times, the
number of dispatches, and the average real I/O service time
of a device, the probability that an I/O operation would be
delayed and the average duration of the delay could be
calculated.